In her speech that started at around 8:40am, Mrs May said Europe must not let “rigid institutional restrictions” on working with a non-member state or “deep-seated ideology” undermine existing cooperation.

May has insisted Brexit “should not and will not” change Britain’s security obligations to Europe.

Theresa May said Britain has “shared the pain and heartbreak” of terror attacks across Europe, as she mentioned the attacks in Westminster in Manchester last year.

She said: “These people don’t care if they maim Parisians, Berliners or Londoners.” “When these atrocities occur, people look to us as leaders to provide the response. We must ensure nothing prevents us from fulfilling our first duty as leaders to protect our citizens.”

She is also expected to say: “Europe’s security is our security.

“And that is why I have said that the United Kingdom is unconditionally committed to maintaining it.

“The challenge for all of us today is finding the way to work together, through a deep and special partnership between the UK and the EU, to retain the co-operation that we have built and go further in meeting the evolving threats we face together.”

“I recognise there is no existing security agreement between the EU and a third country that captures the full depth and breadth of our existing relationship.

“But there is precedent for comprehensive, strategic relationships between the EU and third countries in other fields, such as trade. And there is no legal or operational reason why such an agreement could not be reached in the area of internal security.”

Mrs May will highlight the successes that the UK and European security agencies have had in stopping terrorism.

The Prime Minister is also expected to cite the example of Operation Triage, where police in the UK worked with Europol and the Czech Republic to break a trafficking gang involved in labour exploitation.

Mrs May will also call for unity during a testing time as Britain leaves the EU.